Hydraulic actuators have long been used for motion control including, for example, active suspension systems which apply energy to the active suspension in response to various loads applied to a wheel and/or associated vehicle body. In order to achieve a desired level of performance, an active suspension system needs to have energy either already present or capable of being provided at an appropriate time. In the case of hydraulic systems, the necessary energy corresponds to a necessary hydraulic pressure and flow. One common approach used in hydraulic actuation systems, as well as hydraulic active suspension systems, to ensure that energy is applied in a timely manner is to use a continuously operating pump to provide a desired hydraulic pressure and flow. These types of systems control the fluid flow and pressure provided by the continuously operating pump either by controlling the displacement of the pump and/or using one or more electronically controlled valves to control the fluid flow and pressure from the pump to the actuator. Some systems, especially systems including fixed displacement pumps, use valves to by-pass the actuator. However, it should be noted that in some hydraulic systems, a speed of the pump may be adjusted to increase or decrease the hydraulic flow volume and/or pressure.
Hydraulic actuator systems and hydraulic suspension systems are typically powered using a hydraulic actuator associated with a remotely located hydraulic power source used to transfer hydraulic fluid to and from the actuator via an arrangement of hydraulic hoses or tubes. Hydraulic power sources may include various components including, for example, an electric motor and pump assembly as well as a fluid reservoir.